Added __USE_ISOC99 #define to mac builds.
Added __USE_ISOC99 #define to mac builds.

This is necessary to work around problems with boost/numeric/interval.hpp. Formerly we had been requiring that people building on OS X did this manually, whereas the Gaffer build does it automatically - this brings the two into line.

goddardl/cortex,hradec/cortex,lento/cortex,hradec/cortex,appleseedhq/cortex,davidsminor/cortex,goddardl/cortex,davidsminor/cortex,davidsminor/cortex,goddardl/cortex,lento/cortex,appleseedhq/cortex,lento/cortex,appleseedhq/cortex,hradec/cortex,davidsminor/cortex